The optical bleaches to which this invention is directed are described in the Keller et al. U.S. Pat. No. 2,784,183, issued Mar. 5, 1957. These compounds fluoresce in the bluegreen range and impart to substances coated or incorporated therewith a bright "white" appearance. The compound: 2-(stilbyl-4")-(naphtho-1'2':4,5)-1,2,3-triazole-2" sulfonic acid sodium salt, as described therein, is a commercial product sold under the trademark "TINOPAL RBS". As such, improvements in manufacturing procedures are of great economic importance.
The general scheme for the synthesis of this compound is according to the reaction scheme of the equations in Table 1.
TABLE I ______________________________________ CHEMISTRY ______________________________________ A. Diazotization ##STR1## ##STR2## B. Coupling ##STR3## ##STR4## ##STR5## C. Oxidation ##STR6## ##STR7## ______________________________________
In the coupling reaction (B) the diazonium salt was neutralized from the acidic diazotizing reaction, cooled to about 10.degree.-15.degree. C. and added to an excess of 2-naphthylamine-1-sulfonic acid (Tobias acid) originally in the presence of lutidine, and recently 2-butoxyethanol, Butyl Cellosolve (C.sub.4 H.sub.9 --O--CH.sub.2 CH.sub.2 OH). Yields in the range 70-80% based on PASSA were obtained.
Coupling reactions that are reported in the prior art are conducted differently than in the present invention. For example, in Geigy, British No. 781,821 and Keller, U.S. Pat. No. 2,784,183, the diazo compound is added to a neutralized solution of the coupling agent. This mode of addition is the one customarily used for a coupling reaction to avoid decomposition of the diazo compound under basic conditions and is opposite to that employed in the present invention. Crounse, U.S. Pat. No. 3,689,425, also adds the diazo compound to the coupling agent as does Marschall, U.S. Pat. No. 3,157,644. Marschall, however, destroys the excess nitrous acid in the diazo with sulfamic acid before adding it to the coupling agent. Fujita, U.S. Pat. No. 4,263,624, Alder, U.S. Pat. No. 4,141,903 and Fleck, U.S. Pat. No. 4,167,629 also report coupling reactions. Fujita and Alder employ the standard addition of diazo solution to the neutralized coupling agent. Fleck adds the neutralized coupling agent to the diazo solution which is strongly acidic. This is in contrast to the present invention where the diazo solution is neutralized prior to adding the coupling agent to it. In addition to the above disclosures on the coupling process, the use of alcohols including butyl Cellosolve as solvent and the use of sodium acetate or carbonate as neutralization agent are mentioned in these last three references. Only Geigy and Keller refer to the production of 2-stilbylnaphthotriazole compounds. The other references report various other coupling reactions.